Geochemistry in a Reservoir and Production Perspective

Abstract

Reservoir geochemistry has proved to be a valuable approach to constrain reservoir model and predict, or evaluate, potential production problems at an early stage in field development. The now classical reservoir geochemistry methodology is based on the interpretation of compositional variations detected in oil or in soluble bitumen from rock reservoir including gross composition and molecular distribution1,2. These variations are usually related to long term geological processes (biodegradation, water-washing, gas solubilization, thermal sulfate reduction, and thermodynamic disequilibrium)3 which alter the original oil composition inherited from the source-rock. In the presence of efficient permeability barriers which prevent homogenization of the fluids by diffusion processes, the compositional variations can be eventually preserved over geological time. Additionally, by the possibility of combining molecular information on test fluid samples and reservoir rock organic content, reservoir geochemistry can lead to a better evaluation of the reservoir fluid stability and detection of internal reservoir heterogeneity such as bitumen rich levels or pyrobitumen occurrence4–5. Because such molecular information is available from the first obtained test oil or core samples, the occurrence of difference in fluid compositions across a field can be detected in advance of any production. Such information can be incorporated into the appraisal of the reservoir communicability and taken into account in the very first development scheme. Additionally, molecular fingerprints from production fluids can be used to calculate the relative contribution of individual reservoir in case of multi-reservoir single completion1 or oil mud contamination. In parallel to this pragmatic methodology, a new reservoir organic geochemistry approach is emerging. The objective is to integrate the reservoir fluid in the geological dimension of the petroleum system. This is done through the understanding, the formalization and the modeling of the different geological processes which alter the original composition of the fluid. The aim is to calculate the effect, at the molecular level, of the mechanisms potentially responsible for the modifification of the fluid composition (thermodynamic equilibrium, thermal cracking, water-washing and biodegradation), in the context of the geological environment. Using these simulations, it becomes possible to identify the process(es) which is (are) responsible for the recognized fluid composition and eventual heterogeneity. In this respect, reservoir geochemistry becomes predictive in term of fluid distribution at the reservoir and regional scale.